Origin of the Reactivity of Allyl Chloride and a-Chloroacetaldehyde in S_N2 Nucleophilic Substitution Reactions: A Theoretical Comparison

A comparison of the molecular orbitals of the transition state for the S_N2 nucleophilic substitution of chloride ion on allyl chloride and a-chloroacetaldehyde was made by employing ab initio calculations. These data suggest that the enhanced rate of nucleophilic substitution for both reactions is the result of a low lying “allylic” bonding interaction present in the transition state. The origin of this relatively low lying molecular orbital is traced back to a three molecular orbital four-electron interaction between the nucelophile and the σ-π conjugated bonding and antibonding molecular orbitals comprising the σ_CCland σ*_CClbonds of the substrate.